1. Field of the Invention
The present invention relates to a backlight unit and a display device, and more particularly to the backlight unit and the display device suitable for application to a liquid crystal display device or the like having a large display screen.
2. Description of the Related Art
Liquid crystal display devices have a large display screen and reduced weight, thickness and power consumption as compared to a CRT (Cathode-Ray Tube), and accordingly have been used widely for a television receiver and various displays in addition to a light-emitting display such as a PDP (Plasma Display Panel), for example.
A liquid crystal display device includes a liquid crystal sealed between two transparent substrates having various sizes, and a voltage is applied to change a direction of liquid crystal molecules so that optical transmittance is changed to optically display a predetermined image and the like. Since the liquid crystal is not a light-emitting material, the liquid crystal display device includes a backlight unit serving as a light source at the rear of a liquid crystal panel, for example. The backlight unit is configured to have a primary light source, a light guide plate, a reflection film, a lens sheet or diffusion film, and the like so that display light is supplied over the whole plane of the liquid crystal panel. In the past, a fluorescent tube is sealed with mercury or xenon to obtain a CCFL (Cold Cathode Fluorescent Lamp) as a primary light source for the backlight unit. However, CCFL may have such disadvantages as low light-emission luminance, short life, or insufficient luminance uniformity due to existence of a low luminance area on the cathode side.
An LED backlight unit configured to obtain white light by arraying two-dimensionally a large number of light emitting diodes (hereinafter, referred to as LED (Light Emitting Diode)) of three primary colors of light, which are red, green and blue, on the rear side of the diffusion panel has been attracting attention instead of the CCFL. Such LED backlight unit has low power consumption and is capable of high luminance display on a large-size liquid crystal panel.
Since there is a negative correlation between the temperature of LED and the luminous efficiency thereof, cooling and heat uniformity of the LED may be required. Typically, the temperature rises higher in a central part of the backlight unit than in a periphery portion thereof. Since the luminous efficiency of the LED is generally low and a large amount of heat is generated, an efficient heat discharge system may be required. As a characteristic of LED, light is emitted in one direction, which is different from CCFL, and therefore the heat can be dissipated in the direction opposite to the direction in which light is emitted.
Accordingly, the heat has been dissipated in the past using a bulk material having a certain volume with large heat capacity for the cooling and heat uniformity (for example, refer to Japanese Unexamined Patent Application Publication No. 2005-317480). In the patent document, aluminum (Al) having sufficient heat conduction is used for the bulk material (heat dissipation plate 24) to transmit the heat in the central part of the backlight unit to a heat sink, and further a heat pipe 25 made of copper is used. The heat pipe 25 also contributes to the heat uniformity.